This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the presently described embodiments. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present embodiments. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In order to meet consumer and industrial demand for natural resources, companies often invest significant amounts of time and money in finding and extracting oil, natural gas, and other subterranean resources from the earth. Particularly, once a desired subterranean resource such as oil or natural gas is discovered, drilling and production systems are often employed to access and extract the resource. These systems may be located onshore or offshore depending on the location of a desired resource.
Floating drilling platforms are sometimes used for offshore drilling operations and include a hoisting system for raising and lowering equipment, such as a drill string, to a subsea wellsite. Because these platforms float at the surface of the water and are not anchored to the seabed with legs, the platforms can vertically rise and fall (i.e., heave) with waves in the water. Heave compensation can be used to counteract the vertical heaving motion and reduce movement of the drill string or other hoisted load with respect to the seabed.
Various types of heave compensators have been used in an effort to maintain a constant weight on bit for a hoisted drill string and reduce deviation of the drill string with respect to the seabed as the drilling platform rises and falls with the waves. Simple heave compensators acting as shock absorbers have been provided between traveling blocks and drill strings hoisted with a drawworks system. Active heave compensation has also been used, in which heaving motion of the drilling platform is measured and used to actively control the position of the drill string.
As operators have moved to deeper waters and deeper wells, the weight of the equipment to be hoisted by offshore rigs (e.g., drill strings, casing strings, and wellhead equipment) has increased. Multi-part block-and-tackle arrangements have been used with drawworks for hoisting on drilling rigs, in which hoisting lines are reeved through sheaves of crown and traveling blocks to provide a mechanical advantage. One approach to increasing the hoisting capabilities of such arrangements is to add more lines and sheaves and increase the size of the hoisting lines. Drilling platforms have also been provided as hydraulically driven “cylinder rigs,” which use large hydraulic cylinders instead of drawworks. The hydraulic cylinders in such rigs can provide both the main hoisting function and a heave compensating function.